The man, who holds Estonian and Russian citizenship, was reportedly detained on 8 February at the border checkpoint with Belarus. Latvian security authorities said he was following instructions from Russian special services.
He is facing charges of aiding and abetting a hostile country to act against Latvia.
The monument dedicated to the partisan leader Adolfas Ramanauskas-Vanagas in Merkinė was daubed with red paint on 29 January, in a second similar incident since the monument was built.
In 2018, the Seimas adopted a declaration recognising Ramanauskas-Vanagas as the head of the Lithuanian state during the occupation period. Ramanauskas-Vanagas was the highest Lithuanian state official alive from the death of partisan leader Jonas Žemaitis-Vytautas in captivity on 26 November 1954 until his own assassination in a KGB prison in Vilnius on 29 November 1957.